Topband: Measuring Common Mode Chokes

Jim Thomson jim.thom at
Fri Dec 20 01:19:40 EST 2019

Date: Wed, 18 Dec 2019 23:46:56 +0000
From: Chuck Hutton <charlesh3 at>
To: "topband at" <topband at>
Subject: Topband: Measuring Common Mode Chokes

<In the past, I have simply used my N2PK VNA to measure impedance of the choke by connecting the shield of the coax to the VNA ports.

<Recently I've been discussing common mode chokes with others who have a different methodology.
<They prefer to do a transmission test through the choke and report the "common mode rejection".
<This is done by placing a crossover cable between the VNA output and the choke. The choke output is connected in a normal fashion (center to center, shield to shield) to the VNA input.

<This does not seem ideal to me.
<First, the choke is being driven in differential mode rather than common mode.
<Second, the measurement depends on (varying) isolation between the coax center and shield. So it's not truly common mode rejection.

<Am I on thr right track?
<A handful of Googles has not netted me any clear summary of test methodology for reporting CMRR. I find a small number of tests reporting impedance.


##  I  measure the  actual common  mode  current  with a MFJ-854  clamp  on  RF  ammeter.   Good  from less than 1ma  to  3A.   Just  clamps  around  the 213-U  coax. 
Then when  swapping CM  chokes,  any changes or  improvements, or  worsening is  readily  apparent  at  a glance.   I measure  In  several places  on  the  coax  run, and  all  are well  marked.      Switchable In  5  ranges.

Jim   VE7RF

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